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Old 04-21-2018, 03:35 PM   #1
CC69Rat
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Big Block - possible timing issue (?)

I have a 68 with a fairly hot BBC. It's a 396, .0060 over. 427 Eagle crank, 290 heads, headers, Magnaflows with X pipe, RPM intake, KB hyper pistons, billet timing gears, Voodoo 60204 cam, roller tip rockers. TH400, 3.73. 3200 stall. HEI dist. Vac advance connected. , 8.2 wires. ACDelco Rapid Fire platinum #4.. It's currently sitting at 13 deg BTDC.

When you start the truck. Cold start it wants to crack and pop through the exhaust. Until you get up to speed (20-30mph) it will smooth out but just in that lower RPM range, off a dead stop it is just an occasional pop through the exhaust.

Your thoughts? I am thinking either timing or advance issue.
All input welcome. Help me out..
Thanks
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Old 04-21-2018, 03:38 PM   #2
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Re: Big Block - possible timing issue (?)

Double check your firing order to make sure you don't have a wire out of order. Make sure the dist cap isn't cracked or a wire is grounding itself out. Just a few initial thoughts.
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Old 04-21-2018, 03:51 PM   #3
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Re: Big Block - possible timing issue (?)

When my plugs get gas fouled from a carb problem they I get exhaust pop like that.
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Old 04-21-2018, 04:05 PM   #4
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Re: Big Block - possible timing issue (?)

We did have the choke (elec) clocked way rich. We have since changed that.. and it doesn't seem as rich anymore.

Could it just be gas fouled plugs?

New dist cap, new Taylor 8.2mm wires
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Old 04-21-2018, 04:16 PM   #5
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Re: Big Block - possible timing issue (?)

Pull 2 from each side and take a good look. I've done this drill in the past ... if they are even off a little I change them all. Especially if you are running a cold plug.

Last edited by OBS454; 04-21-2018 at 04:18 PM. Reason: forgot a word
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Old 04-21-2018, 06:49 PM   #6
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Re: Big Block - possible timing issue (?)

That’s a lot of cam with only 13 degrees.
A little reading for you.

http://www.badasscars.com/index.cfm/...d=76/prd76.htm

You didn’t say what carb.
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Old 04-21-2018, 11:13 PM   #7
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Re: Big Block - possible timing issue (?)

I'm no pro at timing, for sure.

It's a custom built 3310-4. Speced for this truck and setup.
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Old 04-21-2018, 11:21 PM   #8
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Re: Big Block - possible timing issue (?)

Follow that link and become a pro.
I had a 396 years ago. Ran the timing locked out at 26 degrees, with vac adding another 16. Always started hot, never pinged, hauled butt, would chirp the hides into high gear. That motor liked lots of timing.
Good choice of carb.
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Old 04-21-2018, 11:22 PM   #9
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Re: Big Block - possible timing issue (?)

If it had SMOG equipment like the AIR pump, I trust that it is not there? Those things pumped air into the exhaust to further oxidize it while cold, and sometimes caused some noise. Otherwise, I would accuse lean mixture. Do you have a choke, set up properly?
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Old 04-22-2018, 05:26 PM   #10
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Re: Big Block - possible timing issue (?)

I think we have it figured out. The electric choke was 'clocked' about 130 deg too far rich. On a cold start, the choke was staying closed and as you worked the throttle it would set the choke cam (red, on the choke mech that keeps the choke closed) making it stick closed when the engine was warming up. It was so rich it would burn your eyes. As I turned that choke body around the choke flap started to open.

Anyway, I have both idle screws at about 1.75 turn out. Choke is much better, idle needed adjusted way down, and we have it about 20 - 22 Deg fast now

What a difference. No popping, the throttle is super crisp. About 16psi on a vac gauge on the back manifold vacuum port.

What do you guys think? Should I just drive it for now and see? It seems much healthier and happier.
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Old 04-22-2018, 06:03 PM   #11
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Re: Big Block - possible timing issue (?)

Drive it!
I like to run a ground wire from one of the 3 screws that hold the choke thermostat on, to the spot on the engine where the neg battery cable attaches, to insure ground to the choke. I do the same from one of the vac advance screws on the distributor.
Sounds a little extreme, but everything works best when properly grounded...
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Old 04-22-2018, 08:57 PM   #12
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Re: Big Block - possible timing issue (?)

Quote:
Originally Posted by CC69Rat View Post
I think we have it figured out. The electric choke was 'clocked' about 130 deg too far rich. On a cold start, the choke was staying closed and as you worked the throttle it would set the choke cam (red, on the choke mech that keeps the choke closed) making it stick closed when the engine was warming up. It was so rich it would burn your eyes. As I turned that choke body around the choke flap started to open.

Anyway, I have both idle screws at about 1.75 turn out. Choke is much better, idle needed adjusted way down, and we have it about 20 - 22 Deg fast now

What a difference. No popping, the throttle is super crisp. About 16psi on a vac gauge on the back manifold vacuum port.

What do you guys think? Should I just drive it for now and see? It seems much healthier and happier.
Do you have an aftermarket cam in it? I'd expect to see higher vacuum. I have about 19" Hg of vacuum at warm idle.
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Old 04-22-2018, 09:15 PM   #13
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Re: Big Block - possible timing issue (?)

Yeah, it has a pretty nasty cam in it. Lunati Voodoo. 60204 I think is the part number.

Where is your timing to pull 19 psi at warm idle?
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Old 04-23-2018, 12:39 AM   #14
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Re: Big Block - possible timing issue (?)

I think it's about 8, but I'd have to check. It's an "RV" cam, whatever that is, that was installed about 25 years ago when the engine was rebuilt. Nothing nasty, by any means. I didn't own the truck when the cam was installed; it was my wife's father's then. It has reasonable off the line torque, but is no stump puller. Though it will burn rubber if I cob it.
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Old 04-23-2018, 10:46 AM   #15
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Re: Big Block - possible timing issue (?)

Quote:
Originally Posted by geezer#99 View Post
Follow that link and become a pro.
I had a 396 years ago. Ran the timing locked out at 26 degrees, with vac adding another 16. Always started hot, never pinged, hauled butt, would chirp the hides into high gear. That motor liked lots of timing.
Good choice of carb.
Just had a 427 on the engine dyno, stock everything. Turns out it made the exact same power with 31, 32, and 34 degrees (well, within 2-3 repeatable anyway). I was surprised.

Down low they idle better and make more vacuum, but big blocks don't actually appear to like that much timing. That might change if you go open port to closed port, which changes the burn (mine's the old 1969 427/390).
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Old 04-23-2018, 09:29 PM   #16
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Re: Big Block - possible timing issue (?)

Quote:
Originally Posted by davepl View Post
Just had a 427 on the engine dyno, stock everything. Turns out it made the exact same power with 31, 32, and 34 degrees (well, within 2-3 repeatable anyway). I was surprised.

Down low they idle better and make more vacuum, but big blocks don't actually appear to like that much timing. That might change if you go open port to closed port, which changes the burn (mine's the old 1969 427/390).
How much power does the L36 make Dave? I have one for my truck that has a Perfomer RPM on it along with a 750 double pumper and Comp nitrous hydraulic roller (224-236 @ .050 on 113 LSA).

FWIW, set your initial timing to where you want and tweak the distributor to get the max that you want (34-36 would be good but Dave's #'s show that might be too much) Then hook the vacuum up to a MANIFOLD source. The extra timing at part throttle and cruise will improve efficiency, mileage, and the motor will run cooler.
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Old 04-24-2018, 11:14 AM   #17
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Re: Big Block - possible timing issue (?)

How'd you know it was an L36? We made 402 against a rated 390. That's stock blueprint factory cam, stock everything. It was punched out to 0.060 over, which is 3%, and made 3% more power than stock, so by that metric was -perfect-. 498 lb-ft but just couldn't crack 500!

Then I'll stick an air cleaner, iron exhaust manifolds, accessories and mufflers and it'll make 275 net at the wheels or something like that!

It breaks up at the end of a long run when it sucks the float bowl dry, so we're trying to find a bigger needle and seat. Worked OK at 6.5psi with an electric pump but the mechanical couldn't keep the bowl full with the stock needle and seat. That would have been a bear to sort out in the car!
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Old 04-25-2018, 08:51 AM   #18
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Re: Big Block - possible timing issue (?)

You said it was 390 hp in your post.

Mine is punched .030. Intake, carb, roller cam, good valve job with Pro-Flo Manley valves, Sanderson 3” outlet manifolds. Hoping for 350 rwhp, 500+ lb/ft. But actual output not too big of a concern. Should be a hoot with 200 4R and 3.73 behind it in my 69 short step.

I bought Chevelle deep groove pulleys and a ‘70 Z/28 dual snorkel air cleaner for truck motor.

I have another 427 put together for my ‘Vette. Little more cam (230/236 @ .050) with open chamber heads so knocks compression down a point but they are ported and runn8ng 2” headers. Was in a boat that ran 56 mph with 330hp Merc 454 and ran 68 with the .060 427. Should be a legit 450 hp at crank I’d say.
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Old 04-25-2018, 09:48 AM   #19
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Re: Big Block - possible timing issue (?)

Yup, the factorycams (which is what I'm running) are pretty darned tame (211/214 or similar), but I figured GM put a lot of time into driveability and vacuum, rather than max power, and that's what I care about, so that's what I went with!

I know in my case the motor has the same casting number as the COPO Camaro blocks and the LS-6 Chevelle, so there is HIGH demand to use the blocks in clones!
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Old 04-25-2018, 07:42 PM   #20
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Re: Big Block - possible timing issue (?)

The intake is one of the biggest power killers. Mine is from a '69 Corvette so might be worth some $$$ to somebody, but I'm more interested in running it!
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Old 04-26-2018, 08:26 AM   #21
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Re: Big Block - possible timing issue (?)

390, 391 whatever it takes.

Glad to see there's still some BBC people in the crowd. I like the LS stuff too I just don't know a lot about them. Pop is an old drag racer so I grew up learning the Gen 4 stuff. Just what I've always used. If I ever do another one it will be a Gen2 Camaro, race car. LS , 6 speed autox setup.
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